Photometric means have been used to perform chemical analysis on fluids for many years. Indeed photometers have now become an integral part of most chemical analysis equipment; particularly automatic analytic equipment. These instruments use various means to isolate the test sample into a specific region for measurement. Usually a rigid container called a cuvette, which is transparent to radiation within the frequency range required for the analysis, is used. The cuvette is subjected to radiation and the intensity of the radiation transmitted through the cuvette is monitored by a photocell.
One of the main advantages of an automatic instrument is in the reproducibility of its results. Yet the use of a cuvette into which the fluid to be sampled, buffer solutions, and necessary reagents are deposited, introduces possibilities of error and contamination which limit the reproducibility of results obtained from such instruments. There are a number of reasons for this. First, not only must the sample fluid and buffer solution be introduced into the cuvette while it is in the instrument, but also all the reagents used in the analysis must be introduced at that time. The more fluids that must be transferred by the instrument, the greater the possibility of error and of contamination. Second, the cuvette and the sample fluid contained in it are normally identified only by the fact that the cuvettes are kept in register throughout the instrument. While this is generally sufficient for the operation of the instrument, a positive identification is much to be preferred, especially when such an identification can be used to control the operation of the instrument. Third, a cuvette is usually open to the air and, therefore, susceptible to contamination. Even if this problem is overcome, the cuvette has to be cleaned after each use or the next samples deposited in it will become contaminated. This is an expensive and time consuming procedure which could profitably be eliminated. Finally, the use of a cuvette is a very inefficient way to utilize sample fluid since more fluid than is actually necessary for the analysis must be deposited in the cuvette.
Most of these problems can be overcome if, instead of using a cuvette, a deformable container such as that used in the Analytic Test Pack described and claimed in application Ser. No. 545,494 is used. In this pack, the reagents are prepackaged in this container so that only the sample fluid and necessary buffer solution need be transferred to the pack when it is in the instrument. The container is completely closed and disposable so that the probability of contamination is kept to a minimum, and cleaning is unnecessary. In addition, the test to be run can be positively identified on the container, and this identification can be used to program the operation of the instrument at each stage of the analysis.
The use of a deformable container, however, still leaves the problem of using the fluid within the container most efficiently. In order to have a reproducible photometric analysis of the fluid contained in the sample cell, some means must be provided for insuring that the length of the sample cell presented to the photometer is the same in each case. This length should be maximized, since the sensitivity of a photometer is directly proportional to the length of the path which the light takes through the sample. In addition, in the case of a sample cell made from a film, the area of the sample cell presented to the photometer should also be maximized so that the effect of reflection from imperfections in the surface of the film can be minimized. Competing with the advantage of maximizing the dimensions of the sample cells are the advantages realized from minimizing the amount of fluid contained in the sample cell. This is important both from the point of view of cost, if the amount of reagent required is reduced, and in those cases where there is very little sample fluid available.